linux-kernel - Re: [PATCH] sched/debug: initialize sd_sysctl_cpus if !CONFIG_CPUMASK

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Message-ID: <20190131134457.caj7vaestug4q2l3@gabell>
Date:   Thu, 31 Jan 2019 08:44:58 -0500
From:   Masayoshi Mizuma <msys.mizuma@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
        Joe Lawrence <joe.lawrence@...hat.com>
Subject: Re: [PATCH] sched/debug: initialize sd_sysctl_cpus if
 !CONFIG_CPUMASK_OFFSTACK

On Wed, Jan 30, 2019 at 09:14:00PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 29, 2019 at 10:12:45AM -0500, Masayoshi Mizuma wrote:
> > From: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
> > 
> > register_sched_domain_sysctl() copies the cpu_possible_mask into
> > sd_sysctl_cpus, but only if sd_sysctl_cpus hasn't already been
> > allocated (ie, CONFIG_CPUMASK_OFFSTACK is set).  However, when
> > CONFIG_CPUMASK_OFFSTACK is not set, sd_sysctl_cpus is left uninitialized
> > (all zeroes) and the kernel may fail to initialize sched_domain sysctl
> > entries for all possible cpus.
> > 
> > This is visible to the user if the kernel is booted with maxcpus=n, or
> > if ACPI tables have been modified to leave cpus offline, and then
> > checking for missing /proc/sys/kernel/sched_domain/cpu* entries.
> > 
> > Fix this by separating the allocataion and initialization, and adding
> > a flag to initialize the possible cpu entries while system booting only.
> > 
> > Signed-off-by: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
> > Reviewed-by: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
> > Tested-by: Syuuichirou Ishii <ishii.shuuichir@...fujitsu.com>
> > Tested-by: Tarumizu, Kohei <tarumizu.kohei@...fujitsu.com>
> > ---
> >  kernel/sched/debug.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > index de3de997e245..9c6637f3e21d 100644
> > --- a/kernel/sched/debug.c
> > +++ b/kernel/sched/debug.c
> > @@ -310,6 +310,7 @@ static struct ctl_table *sd_alloc_ctl_cpu_table(int cpu)
> >  
> >  static cpumask_var_t		sd_sysctl_cpus;
> >  static struct ctl_table_header	*sd_sysctl_header;
> > +static int register_sched_domain_sysctl_on_boot = 1;
> >  
> >  void register_sched_domain_sysctl(void)
> >  {
> > @@ -344,9 +345,12 @@ void register_sched_domain_sysctl(void)
> >  	if (!cpumask_available(sd_sysctl_cpus)) {
> >  		if (!alloc_cpumask_var(&sd_sysctl_cpus, GFP_KERNEL))
> >  			return;
> > +	}
> >  
> > +	if (register_sched_domain_sysctl_on_boot) {
> >  		/* init to possible to not have holes in @cpu_entries */
> >  		cpumask_copy(sd_sysctl_cpus, cpu_possible_mask);
> > +		register_sched_domain_sysctl_on_boot = 0;
> >  	}
> >  
> >  	for_each_cpu(i, sd_sysctl_cpus) {
> 
> I change it like the below. By keeping the initial value 0 it can go
> into .bss instead of .data.

Great, thanks!
Should I re-post the patch as v2?

- Masa

> 
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -315,6 +315,7 @@ void register_sched_domain_sysctl(void)
>  {
>  	static struct ctl_table *cpu_entries;
>  	static struct ctl_table **cpu_idx;
> +	static bool init_done = false;
>  	char buf[32];
>  	int i;
>  
> @@ -344,7 +345,10 @@ void register_sched_domain_sysctl(void)
>  	if (!cpumask_available(sd_sysctl_cpus)) {
>  		if (!alloc_cpumask_var(&sd_sysctl_cpus, GFP_KERNEL))
>  			return;
> +	}
>  
> +	if (!init_done) {
> +		init_done = true;
>  		/* init to possible to not have holes in @cpu_entries */
>  		cpumask_copy(sd_sysctl_cpus, cpu_possible_mask);
>  	}